1. Field of the Invention
The present invention provides a method for stabilizing brightness of a cold cathode fluorescent lamp, and more particularly, a method for stabilizing brightness of the cold cathode fluorescent lamp according to feedback of a lower current of two ends of the cold cathode fluorescent lamp.
2. Description of the Prior Art
Regarding to a cathode ray tube (CRT) monitor, a flat panel display (FPD) monitor has incomparable advantages, such as low power consumption, no radiation, small volume, etc., so that the FPD monitor has become a substitute for the CRT monitor. As FPD technology advances, prices of FPD monitors are reduced, which makes FPD monitors more popular. However, a FPD monitor with large window size must include a high-efficiency cold cathode fluorescent lamp, or CCFL. A drive circuit of the CCFL includes factors greatly affecting stability of the CCFL when transforming electricity into light. The factors, such as an input current degree, outside temperature, triggering waveforms, lamp size, inner gases of the CCFL, and even distance between adjacent devices, generate a complex non-linear response in the CCFL, and change the brightness of the CCFL. Moreover, in order to reduce electromagnetic interference, the CCFL is covered by a conductive layer, which is coupled to a system ground. Therefore, between the CCFL and the system ground there is a significant parasitic capacitance.
Please refer to FIG. 1 and FIG. 2. FIG. 1 illustrates a schematic diagram of a prior art light source device 10 of a FPD monitor, while FIG. 2 illustrates a schematic diagram of a thermometer effect. In FIG. 1, a power controller 100 provides alternating currents for a CCFL 104 through a transformer 102. The CCFL 104 generates light corresponding to different current levels, and feeds a current of one end of the CCFL 104 back to the power controller 100. As shown in FIG. 1, one end of the CCFL 104 is coupled to a high alternating current and the other end of the CCFL 104 is coupled to a system ground through a resistor 108, which forms an electromagnetic field (EMF) gradient. The EMF gradient in the CCFL 104 makes the CCFL 104 brighter in one end and darker in the other end, which is due to the thermometer effect shown in FIG. 2. The thermometer effect is stronger when the CCFL 104 receives a lower degree current. Moreover, as mentioned above, the CCFL 104 is covered by a conductive layer, causing significant parasitic capacitance between the CCFL 104 and the system ground, which is represented by a capacitor 106 in FIG. 1. The capacitor 106 results in current leakage from the CCFL 104 to the system ground, so that the power controller 10 cannot detect the exact current in the CCFL 104.
In order to improve the thermometer effect, the prior art will float the CCFL. Please refer to FIG. 3 and FIG. 4, which illustrate schematic diagrams of a light source device 30 and 40 of FPD monitors. The light source device 30 and 40 can improve the thermometer effect. The light source device 30 provides currents for a CCFL 302 according to a feedback current from a primary end of a transformer 300, while the light source device 40 provides currents for a CCFL 402 according to a feedback current from a secondary end of a transformer 400. The light source device 30 and 40 can reduce the thermometer effect caused by the parasitic capacitance. However, since terminals of the CCFL 302 and the CCFL 402 are coupled to high voltage sources, different feedback points between the light source device 30 and the light source device 40 make currents to be provided for the CCFL 302 and 402 change in response to different stray capacitances. As a result, brightness of the CCFL 302 and 402 cannot be stable.
In short, the prior art light source devices are affected by problems of the thermometer effect, parasitic capacitance, and current leakage, which reduce stability of the CCFL and system efficiency.